Just my two cents...I had seen a video by AAMT in which Claudia Tessier addressed this very issue. Her take on this was that it can be typed as "Blood pressure 100 to 120 over 70 to 80" or "Blood pressure 100-120 over 70-80."
BUT NEVER 100-120/70-80 or 100/70 to 120/80
It's a clarity issue... 